TOOLSHED: GREENHOUSES

Given even the slightest encouragement, gardening tends to progress rapidly from necessary chore to obsessive passion. At some point in this transition you are bound to have cause to consider buying a greenhouse. Do it. With the exception of a new and bigger garden, nothing can so immediately and dramatically extend your horticultural horizons.

The standard greenhouse is 6ft x 8ft, but any sensible gardener will go as big as he or she can afford and accommodate. It will fill up remarkably quickly. When you sow a packet of seeds you don't need a lot of room, but by the time the seedlings are pricked out into three-inch pots it is a different story. Some greenhouses are designed to accept add-on sections, allowing for expansion in line with demand, which seems eminently sensible. For the tightest spaces, a hexagonal design provides a lot of growing room for the area covered and looks natty into the bargain.

A big decision is whether to go for wood or metal. The imprudence of using a framework containing iron is illustrated by the small fortune recently spent on restoring the Victorian palm house at Kew. Iron rusts - and greenhouses are damp places. The majority of greenhouses are now of non-corroding aluminium which is both cheap and virtually maintenance- free. A new 6ft x 8ft one can be your for less than pounds 300. However, its bright, silvery surface tends to disagreeably obtrusive. An acrylic paint finish, available in a variety of neutral colours, spares this embarrassment but bumps up the price considerably.

Wooden greenhouses have the great advantage of being more attractive to look at. The main drawback is that most softwoods rot fairly rapidly unless they are painted or varnished and regularly maintained. The only way to avoid this major inconvenience is to use a naturally durable timber such as Western Red Cedar. This will resist rot for decades without any treatment, and weather to a pleasing silvery grey. Unfortunately, wouldn't you just know it, they cost at least twice as much as an equivalent aluminium model.

Horticultural glass is supplied as standard to most greenhouses. Other possible options are toughened glass (expensive), or various plastics such as polycarbonate, polythene or rigid UPVC. All are less good at transmitting light and have a limited life span so are only worth considering if there is a real risk that children, or vandals, may break the glass.

It always makes sense to look before you buy, and larger garden centres usually have a number of show greenhouses. Check the framework is sturdy, with no big gaps between panels. The door should fit well, open easily and preferably be wide enough for a wheelbarrow. The ridge height should be a minimum of 7ft sloping down to not less than 5ft at the eaves.

The biggest problem with most off-the-shelf greenhouses is inadequate ventilation. The area of the opened vents should be around one sixth that of the floor. For a 6ft x 8ft greenhouse this means a minimum of three: preferably two in the roof and one at lower level, so cool air can be drawn in at the bottom as hot air leaves from the top. Most manufacturers will fit extra vents for a reasonable charge - money well spent.

The shopping list does not stop at the structure itself. You will certainly want staging (wide, worktop-height shelving) along at least one side. This needs to be sturdy and easy to remove. Running mains electricity out to a greenhouse can be an expensive undertaking but it will greatly increase your options. An electric fan heater is the most efficient method of heating a greenhouse and a heated propagator is a great help for raising both seed and cuttings.

Finally, a warning. Putting up a greenhouse can be like struggling with an inscrutable puzzle. You would be wise to get in at least one extra pair of hand and be prepared for a long and frustrating day.